It's illegal to leave children alone here under the age of 14 without having made sufficient provision for their safety (not the UK before anyone jumps on me).
The actual wording on the CYFS website (Child Youth and Family Services) is:
In New Zealand, it is against the law to leave children under 14 without making reasonable provision for their care and supervision. What is considered 'reasonable' takes into account the circumstances in which children are left alone and the length of time they are alone. Parents are required to assess all the circumstances and make sure that any child left alone, or in the care of another child or young person is safe and not in danger.
Up to the age of about 14, most children are not sufficiently mature to be left without adult supervision on a regular basis, or for more than a short time. While it can be tempting for parents to leave children alone for an hour or two after school, at this age there are still many risks. Older children may seem to be reasonably happy and able to entertain themselves, but they may not be able to cope with an emergency. Unsupervised children are at risk from household or cooking accidents, fire, poisoning, unmonitored internet and phone access and playing with weapons. They are also more likely to get into trouble or put themselves at risk of harm without adult supervision and guidance. If an older child is left alone for a short time, parents or caregivers need to make sure the child knows where they are and who they can contact if there is a problem. Parents also need to check that their older child feels confident left alone and that they know what to do if they need assistance.
There's a world of difference between a predictable bus journey with a recognised end (home -> school where you will be checked in and a parent phoned if you have not arrived, or vice versa where non arrival at home after sufficient time to walk from the bus stop should also raise an alarm) and being left for several hours a day or overnight with no supervision.
My children are currently at an age where I don't have to worry about this (7 and 8 and therefore too young to be left alone in any case). As they get older I'll be giving them more and more "supervised freedom" but I won't leave them at home on their own before they are 14 if I can help it. Never say never, an emergency may arise where I have to weigh that sort of problem up. I'd probably take them to work and sit them in the tea room with a kindle than leave them at home alone though. And yes, I do have to juggle work and childcare.
They are already learning how to cook, clean, wash, budget etc - I don't need to leave them alone to teach them basic life skills. Once they get to 14 I'll be extending what they do such that by 16-18, they'll be fine to live independently. I'm just not forcing them to grow up sooner than they have to. They get some supervised freedoms (e.g. playing in the local park out of my sight while I walk the dogs, but with knowing exactly what to do, where is and isn't appropriate to go etc).
This doesn't make me a "smug kept woman", I work 60+h a week.
In reality I can't see a UK law being much different to the one we have in NZ and it doesn't seem to cause as much outrage as people are showing on this thread. It's not a judgment on anyone's ability to parent, just a common sense statement that children under a certain age should have appropriate provision made for their care and safety.